Self-installable switchable antenna

ABSTRACT

A system, method, and apparatus for selecting a set of antennas, for use during operation of a radio system, from a plurality of antennas. The system, method and apparatus may include selecting one antenna that is part of an array of antennas. Then measuring characteristics of radio signals received at the antenna. The selection and measuring of characteristic is repeated for a desired number of antennas in the array. Then, the measurements are combined, and the combinations of antennas are ranked based upon the combined measurement. From the ranking combinations of antennas are selected for use during operation of a radio system.

RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationSer. No. 60/749,401, filed Dec. 12, 2005, entitled “Design of a Systemand Method for Switchable Antenna Selection Based on an OptimzedConfiguration,” and U.S. provisional application Ser. Nos. 60/763,196,filed Jan. 27, 2006, and 60/774,428 filed Feb. 17, 2006, both entitled“Design of a Self-Installable System and Method for Switchable AntennaSelection Based on a Quality Ranking of OFDM and OFDMA Signals” all ofwhich are hereby incorporated by reference in their entirety.

BACKGROUND

1. Field of the Invention

This invention relates to multi-antenna systems for communicationsystems and more particularly to selection of antenna within amulti-antenna system for use in communication systems.

2. Description of Related Art

A conventional wireless communication system facilitates two-waycommunication between a plurality of subscriber units and a wirelessnetwork infrastructure. Typically the wireless infrastructure is fixedand the subscriber units can be fixed or mobile, or a combination ofboth fixed and mobile. A typical environment where a wireless networkoperates includes many structures and obstructions that can block orreflect the wireless signal as it propagates between the subscriberunits and the infrastructure. For example, as the signal propagates itmay be reflected off of walls, people or other obstacles within thewireless environment. As the signal propagates, and is reflected, it mayarrive at a receiver from a different angle than a direct path betweenthe transmitter and receiver.

The wireless environment also changes over time. For example, in awireless LAN, the obstacles within the environment move, such as peoplewalking, or furniture or equipment being moved. In addition, if thewireless receiver or transmitter are mobile, their movement also changesthe location of the obstacle relative to the receiver or transmitter. Asthe wireless environment changes so does the path that a signal takes asit propagates through the environment. The change in propagation pathmeans that it is unknown what direction the signal will arrive at thereceiver from a transmitter.

FIG. 1 is a diagram of an example wireless network 100. As shown in FIG.1, the wireless network includes a transmitter 102 and a receiver 104.In a typical environment where a wireless network 100 operates there arealso obstacles 106. In an outdoor environment, examples of obstacles 106include people, buildings, cars, trees, etc. In an indoor environmentsome examples of obstacles 106 include walls, people, furniture,equipment, people, etc. As noted above, the environment also changesover time and the obstacles 106, transmitter 102, or receiver 104, movein relation to each other.

As illustrated in FIG. 1, as the radio signal propagates between fromthe transmitter 102 and is received at an antenna 120 of the receiver104 it can be blocked, and reflected by the obstacles 106 in thesurrounding environment. The example illustrated in FIG. 1 may bereferred to as a non-line of sight (NLOS) propagation environmentbecause the direct line-of-sight (LOS) path 110 is blocked by one of theobstacles 106. Likewise other instances of the signal propagate throughthe environment, being reflected off of obstacles and traveling variousdifferent paths. Four examples of these “multipath” signals 110, 112,114, and 116 are illustrated in FIG. 1. As shown in the example of FIG.1, the multipath signals can arrive at the receiver 104 from differentdirections.

To account for this unknown direction of the receive signal, manywireless devices use an omni direction antenna. While an omnidirectional antenna can receive signals from any direction, the receivedsignal is not optimal because there is no antenna gain. A directionalantenna has antenna gain over an omni antenna but because it isdirectional it can only receive signals from a particular direction.However, because the wireless environment may be changing a fixeddirectional antenna may not provide adequate performance.

A technique used to enhance the performance of directional antennas isan antenna switching system. A typical antenna switching system includesmultiple antennas, or antenna elements, arranged in a pattern thatenables reception of a signal from any direction. With the antennaswitching system, individual antenna elements may be selected such thatthe antenna “Pointed” in the direction of the received signal isselected for use.

Therefore, there is a need for methods and apparatus to optimize antennaselection in a multi-antenna system.

SUMMARY

The systems and methods have several features, no single one of which issolely responsible for its desirable attributes. Without limiting thescope as expressed by the claims which follow, its more prominentfeatures will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description” one will understand how the features of thesystem and methods provide several advantages over traditional framingtechniques.

In accordance with the invention, it is desirable to optimize an antennasystem diversity in Non Line-of-Sight (NLOS) or Obstructed Line-of-Sight(OLOS) propagation environments when two or more receive antennas areemployed. In other words, it is desirable to optimize spatial diversityfor systems that employ one or more antenna feeds that are operating inheavy multipath fading environments. It is also desirable to providecharacteristics of omni-directional antenna coverage in NLOS or OLOSenvironments while also increasing the system gain. The system isself-installable in customer premises equipment (CPE) or SubscriberStation (SS), especially in NLOS environments where a direct antennaalignment is not possible by optical means. The system is also adaptiveto support antenna alignment for time-varying NLOS or OLOS propagationenvironments.

In one aspect, a method of selecting a combination of antennas from aplurality of antennas includes selecting one antenna that is part of anarray of antennas. Then measuring characteristics of radio signalsreceived at the antenna. The selection and measuring of characteristicis repeated for a desired number of antennas in the array, then themeasurements are combined and ranked. From the ranking a combination, orset of combinations, of antennas are selected for use during operationof a radio system.

The radio signals may be orthogonal frequency division multiplexed(OFDM) signals. Also, measuring characteristics of the radio signalincludes, for example, measuring a degree of correlation of an amble (apreamble, midamble, or a postamble) in the signal to a known amble,combining signals received from two or more antennas and then measuringa degree of correlation of an amble in the combined signal to a knownamble, measuring an error vector measurement, a signal to noise ratio, acarrier to signal ratio, or a signal fade measurement.

In another aspect, an antenna system includes a plurality of antennas.The system also includes a switch matrix in communication with theplurality of antennas and adapted to couple a selected antenna to aswitch output. A radio module is in communication with the switch outputand adapted to receive radio frequency signals. There is a measurementmodule in communication with the radio module, the measurement module isadapted to measure characteristics of the radio frequency signalsreceived by the radio module. And a control module that is adapted toreceive the measured characteristics and to control selection of whichof the plurality of antennas is coupled to the switch output of theswitch matrix.

The control module can select antennas one at a time, or simultaneously,and rank the antennas based upon a combination of the measurements ofthe individual antennas. Then the control module selects a combinationof antennas, or a set of antennas combinations, for use during anoperational phase based upon the rankings. The antenna system can alsoinclude a second switch output, and a second radio module incommunication with the second switch output. In this configuration, thecontrol module selects one antenna, or set of antennas whose signals arecombined, to be communicatively coupled to the first radio module and asecond antenna, or set of antennas whose signals are combined, to becommunicatively coupled to the second radio module. Any desired numberswitch outputs and radios can be used.

Other features and advantages of the present invention should beapparent from the following description which illustrates, by way ofexample, aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example wireless network.

FIG. 2 is a diagram of an example multi-antenna module that can be usedat a receiver.

FIG. 3 is a diagram of a wireless network illustrating aspects of theinvention.

FIG. 4 is a diagram of another example multi-antenna module that can beused at a receiver.

FIG. 5 is a functional block diagram of an embodiment of an antennasystem.

FIG. 6 is a flow diagram illustrating selection of antennas in amulti-antenna array.

FIG. 7 is a set of charts graphically illustrating the relative“performance” of the individual antennas.

FIG. 8 is a set of charts graphically illustrating the relative“performance” of the various combinations of antennas.

DETAILED DESCRIPTION

After reading this description it would become apparent to one skilledin the art how to implement the invention in various alternativeembodiments and alternative applications. However, although variousembodiments of the present invention will be described herein, it is tobe understood that these embodiments are presented by way of exampleonly, and not limitations. As such, this detailed description of variousembodiments should not be construed to limit the scope or breadth of thepresent invention.

It is desirable to optimize an antenna systems diversity in NonLine-of-Sight (NLOS) or Obstructed Line-of-Sight (OLOS) propagationenvironments when two or more receive antennas are employed. In otherwords, it is desirable to optimize spatial diversity for systems thatemploy one or more antenna feeds that are operating in heavy multipathfading environments. It is also desirable to provide characteristics ofomni-directional antenna coverage in NLOS or OLOS environments whilealso increasing the system gain. The system should be self-installablein customer premises equipment (CPE) or Subscriber Station (SS),especially in NLOS environments where a direct antenna alignment is notpossible by optical means. There is also a need for adaptive antennaalignment for time-varying NLOS or OLOS propagation environments.

FIG. 2 is a diagram of an example multi-antenna module 200 that can beused at a receiver. In the example antenna module 200 shown in FIG. 2,there are four directional antennas 202, 204, 206, and 208, eachconfigured to receive or transmit signals in approximately a 90 degreesector. Selection which of the antennas 202, 204, 206, and 208, toactivate during operation of a radio system, such as during reception ortransmission of signals can improve the performance of the radio system.

FIG. 3 is a diagram of a wireless network illustrating aspects of theinvention. FIG. 3 is similar to FIG. 1, with the antenna 120 at thereceiver 104 replaced with the antenna module 200 of FIG. 2. In theexample of FIG. 3, the multipath signals 112, 114, 116, and 118 thatarrive at the antenna module 200, arrive at different antennas 202, 204,206, and 208 of the antenna module 200. Because the multipath signals112, 114, 116, and 118 are typically of differing “quality” it may bepreferred to receive selected versions of the multipath signals, anddiscard the others. For example, the multipath signal 112 is onlyreflected once before it is received at antenna 202, while multipathsignal 116 is reflected five times before it is received at antenna 206.Likewise multipath signals 114 and 118 are reflected twice before beingreceived at antennas 204 and 208 respectively.

Because the multipath signals 112, 114, 116, and 118 each traveldifferent paths they will generally undergo differing amounts ofdegradation or distortion. For example, multipath signal 112 may undergothe least degradation because it is only reflected once. While multipathsignals 114 and 118 are both reflected twice, multipath signal 114 mayundergo less degradation because it travels a shorter overall pathlength than multipath signal 118. And, multipath signal 116 may undergothe most degradation because it is reflected five times. Because themultipath signals arrive at the antenna after undergoing differingamounts of degradation the signals will be of different “quality.” Thus,it may be preferred to only activate selected antennas and deactivateother antennas.

FIG. 4 is a diagram of another example multi-antenna module 400 that canbe used at a receiver. In this example antenna module 400, the antennasystem includes eight directional antennas 402, 404, 406, 408, 410, 412,414, and 416 each configured to receive signals from approximately a 45degree sector. Selection of which of the antennas are active can improvethe performance of a receiver. Each of the antennas in antenna module400 cover a small sector thereby increasing the antenna gain over theantennas in antenna module 200 of FIG. 2. While FIGS. 2 and 4illustrated four antennas and eight antennas, any desired number ofantennas may be used.

Multi-antenna modules, such as illustrated in FIGS. 2 and 4 also providespatial diversity. Using spatial diversity, incoming signals from two ormore spatially diverse antennas are combined mathematically to produce asignal that is of better quality than either of the two individualsignals from the antennas. A selectable antenna array can be used tooptimize spatial diversity while also enabling a user-installableantenna. The selection criteria may be based on choosing an optimalpair, or set, of receive antennas which will maximize the performance ofthe combining scheme.

FIG. 5 is a functional block diagram of an embodiment of an antennasystem 500. The antenna system 500 can support MIMO (multiple-inmultiple-out) technology. The antenna system 500 includes a plurality ofconfigurable antennas 502-508. Although four configurable antennas aredepicted, more or fewer such elements can be used. An antenna switchmatrix 510 communicatively couples selected antennas 502-508 to a radiomodule 512.

The radio module 512 receives radio frequency (RF) signals from theselected antenna 502-508, demodulates the received signal andcommunicates a baseband signal to data interface module 514. The radiomodule 512 also receives baseband signals from the data interface module514, modulates the baseband data and communicates an RF signal to theselected antenna 502-508. The radio module 512 is also communicativelycoupled to a measurement module 516. As described further below, themeasurement module 516 evaluates characteristics of the signal receivedby the radio from the selected antenna For example, the measurementmodule 614 may determine the power level, or degree of correlation, orother characteristic of the received signal.

In one embodiment, during a selection operation of the antenna system,signals from desired antennas 502-508 are communicated to the radiomodule 512. An output of the radio module is in communication with themeasurement module 516. For example, signals from each available antenna502-508 can be communicated, one at a time, to the radio module 512where the signal is communicated to the measurement module 516. Themeasurement module 516 analyzes the received signal to determine a“quality” of the signal. In one example, each antenna signal iscommunicated to the radio module 512 for a period long enough to captureat least an amble (preamble, postamble, midamble) from the signalreceived from the respective antenna. The measurement module 516 thenanalyzes the signal to determine its quality by measuring the degree ofcorrelation of the received amble(s) with known reference amblesignal(s). The amble correlation can be used as a criterion for receivedsignals assigned to the same transmitter, or base station, for rankingthe quality of the antenna feeds. A Base Station Identification (BSID)may be used to make a decision between two received signals with thesame degree of amble correlation, transmitted by different base stationsor peer subscriber stations (SS).

The measurement module 516 communicates the determined signal qualityfor each antenna selected to control module 518. The control module 518communicates with the antenna switch matrix 510 to selective couple adesired antenna to the radio module 512. As described further below, thecontrol module 518 evaluates the signal quality received from themeasurement module 516 for each antenna 502-508 selected. Afterevaluating a desired number of antennas 502-508, the control module 518can select antennas to couple to radio modules 512 and 520 duringoperation. It is noted that the control module 512 also controls theantenna switch matrix to selectively couple the antennas 502-508 to theradio module 512 during the selection phase.

The control module 518 evaluates various combinations of antenna signalsto determine preferred antenna combinations. As described further below,the control module 518 uses various analysis techniques to evaluatewhich combination of antenna signals provides a preferredcharacteristic, for example, the “flattest” response. In one embodiment,the various combination of antenna signals are raked by determining a“measurement” of the combinations' performance relative to othercombinations. Table 1 below is an illustrative example of possiblerankings for five antenna array where it is desired to determine theoptimal combination of two antennas within an array of five antennas. InTable 1, a value closer to “1”represents enhanced performance over avalue closer to “0.”

TABLE 1 Antenna 1 Antenna 2 Antenna 3 Antenna 4 Antenna 5 Antenna 1 0.80.9 0.3 0.1 Antenna 2 0.8 0.3 0.3 0.2 Antenna 3 0.9 0.3 0.2 0.4 Antenna4 0.3 0.3 0.2 0.6 Antenna 5 0.1 0.2 0.4 0.6

The values listed in Table 1, which are merely illustrative, show thatin this example the combination of signals from antennas 1 and 3 arepreferred over the other combinations of antennas. The exampleillustrated in Table 1 evaluated the combinations of only two antennas.In other examples, any number of desired antennas may be combined andthe combinations evaluated for performance.

Returning to FIG. 5, signals from each of the desired antennas 502-508can be sampled, one at a time, and then the samples combined to evaluatethe performance of the combination. In another embodiment, samples frommultiple antennas may be sampled at the same time, and then the samplescombined to evaluate the performance of the combination. For example, ifcombinations of two antennas are being evaluated, then using a secondradio module 520 in addition to the first radio module 512, two antennascan be sampled simultaneously. For example a first antenna 502 signalcan be communicated to the first radio module 512 while at the same timea second antenna 506 signal can be communicated to the second radiomodule 520. The radio signals from the two radio modules 512 and 520 canbe communicated to the measurement module 516 where a quality of the twosignals is determined and communicated to the control module 518. Whilethe example shown in FIG. 5 has only two radio modules, any desirednumber of radio module may be used to acquire a desired number ofsamples simultaneously.

The antenna system 500 can be selectively configured in response tocontrol signals to create different antenna configurations which caninclude different antenna patterns. By selecting one or more antennas502-508, the antenna system can direct or steer the gain of the overallantenna system 500.

A system can require one receive antenna feed like in Multiple In SingleOut (MLSO) or Space Time Coding (STC) 2×1 selection spatial diversityconfigurations or two or more receive feeds like in Multiple In MultipleOut (MIMO) or STC 2×2 receive configurations. During an operationalphase, depending on whether the system requires a single antenna feed ormultiple antenna feeds, the highest-ranking antenna feeds determinedduring the selection phase can be enabled and the other antennas in thearray disabled. If the system is a multiple antenna system, combiningtechniques can be applied to the plurality of signals to form an inputsignal for the receiver.

For example, in a NLOS environment the direction of the best performanceRadio Frequency (RF) energy is unknown due to the multiple reflectionsand/or diffractions. As illustrated in FIG. 3, the “best” performance RFsignals could arrive at the receiver from unpredictable directions.Therefore 360 degrees of antenna coverage is desirable. However, usingan omni directional antenna may not be an optimal solution consideringthe low antenna gain associated with this type of antenna. Therefore,sectorized coverage may be used to help optimize the system gain, usingspatial combining schemes. Returning to FIG. 3, the antenna system 200includes four antennas 202-208, each having a 90 degree horizontalcoverage pattern. The received signals that offer the best spatialcombining performance could be received, for example, on the antennas202 and 206 aligned opposite to each other, or any other combination ofantennas.

System gain can be increased further if more complex antenna systems areemployed. For example, using an eight-antenna system 400 as illustratedin FIG. 4 can provide increased gain. As an example, the best RF signalscould be received on the antennas 404 and 414 each having a 45 degreehorizontal coverage pattern and a corresponding increase in gain.Although the examples illustrated in FIGS. 2 and 4 shown antenna arrayswith each antenna in the array having the same coverage pattern, otherconfigurations are possible. For example, an antenna system can includeantennas with differing coverage patterns such as some elements having90 degree coverage patterns and other antennas having 45 degree coveragepattern, or any other combination of desired coverage patterns.

In addition, the selection phase, or process, may be used periodicallyto account for changes in the propagation environment and the relatedsignal degradation as monitored by an upper layer application (Data LinkLayer). For example, an environment that has a high rate of change mayrequire a higher antenna re-selection rate be used than that required ina fixed environment.

Using the techniques for antenna selection described can significantlyimprove the operation of RF systems operating in heavy NLOS multipathenvironments. For example, if a multiple antenna system is an array of90-degree sector antennas, the gain is typically 10-12 dB higher than asingle omni antenna, based on the H polarity gain difference for thesame vertical beam angle of the corresponding radiation pattern. Itshould be noted that insertion losses in an antenna switch used toselect an antenna could degrade the receiver's related noise figure andtherefore lower the system gain. The expected system gain improvementis:ΔSysGain=G _(H) _(—) _(90deg) −G _(H) _(—) _(OMNI) −L _(S)  Equation 1where:ΔSysGain: is the system gain difference between the SS operation basedon selection of a 90 degree sector antenna and an omni-directionalantenna with the same vertical angle of the corresponding radiationpattern.G_(H) _(—) _(90deg): is the horizontal antenna gain for a 90 degreesectorial antenna based on a same vertical radiation pattern angle,expressed in dBi.G_(H) _(—) _(OMNI): is the horizontal antenna gain for anomni-directional antenna based on the same vertical radiation patternangle, expressed in dBi.L_(S): is the aggregate insertion losses for the operational frequencyof the antenna switch, expressed in dB.

Using Equation 1, in one example where G_(H) _(—) _(90deg)=14 dBi, G_(H)_(—) _(OMNI)=4 dBi and L_(s<)=2dB, the expected delta system gainimprovement is:ΔSysGain>=8 dB

Various other techniques can be used to determine which antenna, or setof antennas to select. For example, one technique is to examine a signalfrom each antenna individually. An amble signal can be compared with aknown amble signal by a correlation process. Then a measure ofcorrelation for each antenna is used to compare performance of theindividual antennas and to select the one, two, or more “best”candidates for selection. If there are M antennas available in thesystem, then there would be M correlation operations.

In one embodiment, cross-correlation of the incoming signal against aknown sequence is performed. The cross-correlation operation results ina peak corresponding to the quality of the incoming amble signal. In theexample where two signals are to be used, the tallest two peaks areused. Another property of the correlation technique is that multipathreflections result in secondary, tertiary, and sometimes othercorrelation peaks. Detection of significant peaks outside of theexpected correlation peak indicate strong multipath and could be used torule out a given element. In one example, selection could be performedby measuring the normalized magnitude of the main correlation peak, thenmeasuring the magnitude of the next largest peak and recording thedifference. Correlation for a single element is performed as follows:

$\begin{matrix}{{y(m)} = {\sum\limits_{n = 0}^{N - m - 1}{x_{n + m}y_{n}^{*}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$where N is the length of the correlation sequence.

The peak value is detected and a normalization process is used to ensurecomparison between elements is on the same scale. This results in asingle numerical value that represents element signal quality based oncorrelation. For single-output applications, the element with thehighest numerical value is selected.

Another technique that can be used to determine which antenna, or set ofantennas, to select combines the received signals from two or moreelements before performing the correlation measurement. The correlationsfrom different combinations are then used to determine the “best”candidate combinations for selection, In this scenario, if there are Mantennas available in the system and N output streams, this requires_(M)C_(N) correlation operations. For example, if there is an eightantenna array, and it is desired to select one two antenna set, thenthere would be 28 correlation operations.

Still other techniques for determining which antenna, or set ofantennas, to select may be based on the time domain signal for a simpledetermination of “best” antenna combination in an array. For example,evaluation various antenna combinations to determine characteristics ofthe combination, such as, average received power of the incoming signal,signal-to-noise ratio (SNR), etc.

In addition, using properties of an Orthogonal Frequency DivisionMultiplex (OFDM) signal, antenna comparisons may be made in thefrequency domain. For example, an amble sequence from each antenna maybe converted to the frequency domain using a discrete Fourier transform(DFT) or some other equivalent process. The quality of the signal maythen be determined using, for example, an error vector magnitudemeasurement, signal-to-noise ratio (SNR), carrier-to-signal ratio (CIR),signal fade measurement, or some other metric or combination of metrics.Two or more “best” candidates can then be selected based on thedetermined quality of the signal. Using this technique, if there are Mantennas available in an array, then there would be M frequency domainmeasurement operations.

As with the above frequency domain signal performance measurements, anamble sequence can be converted to the frequency domain using a discreteFourier transform (DFT) or some other equivalent process. If there are Minput array antennas and N required output streams, N combinations ofthe M input streams are combined before the frequency domain signal ismeasured. The quality of the signal can then be measured, for example,using an error vector magnitude measurement signal-to-noise ratio (SNR),carrier-to-signal ratio (CIR), signal fade measurement, or some othermetric or combination of metrics. Two or more “best” candidates can beselected based on these results. If there are M antennas available inthe array and N required output streams, is requires _(M)C_(N) frequencydomain measurement operations.

For example, if the signal is transmitted during two time frames asillustrated in Table 2:

TABLE 2 Tx Antenna 0 Tx Antenna 1 Time t s₀ s₁ Time t + T −s₁* s₀*the notation for the received signals at the two receive antennas arelisted in Table 3:

TABLE 3 Rx Antenna 0 Rx Antenna 1 Time t r₀ r₂ Time t + T r₁ r₃

In a two-antenna receiver system, the values r_(0.3) are combined torecover the original s₀, s₁ values transmitted. The invention can beused to select a combination of antennas with optimal signals for usewith the space-time receiver. To do this, the amble signals r₀, r₁, fromone element and r₂, r₃ from a second element are combined to get anestimated s₀, s₁ signal. The combing process follows techniquesdescribed by S. Alamouti, in “A Simple Transmit Diversity Technique forWireless Communications,” IEEE Journal on Select Areas inCommunications, Oct. 1998, pp 1451-1458, incorporated by referenceherein.

An error vector magnitude measurement is taken to create a numericalvalue for the quality of the selected element combination. This processis repeated for the other possible array combinations and the highestquality combination (lowest total error vector magnitude) is selected.

To improve the coherence of mixed array antenna signals, a relative feeddelay can be calculated, for example, using the results of the timedomain correlation of each signal or phase detection/correctioncircuitry in the frequency domain. Delay circuitry before the DFT orphase rotation circuitry following the DFT could be used to align thetiming or phase of the signals prior to combining them. For example, ifan antenna system includes a four-antenna array, similar to the arrayillustrated in FIGS. 2 and 5 above, then one or more antennas with“strong” signals can be selected to feed a receiver, and there would bean increase in system gain because directional antennas have more gainthan an omni directional antenna.

In one example, a channel estimate is performed in the frequency domainusing the amble. Timing/phase adjustments are made to the incomingsignals based on phase error measurements. The signal is transformed tothe frequency domain, and parts of the bandwidth under fade are notedfor each antenna. Antenna elements are selected and combined (in thefrequency domain) based on their ability to complement each other. Thefade measurement process includes measuring the magnitude of the signalin each frequency bin and storing a vector containing the values foreach element. Next, the antenna combinations are compared by summing thevectors for two candidates and calculating the variance for theresulting sum vector. In the case of a 4×4 array, the combination of twoelements with the lowest variance is selected. This results in twostreams of two combined elements.

For a four-element array (using A . . . D to designate each of the fourelements), the following comparisons are made:min(var(A+B),var(A+C),var(A+D),var(B+C),var(B+D),var(C+D))  Equation 3where “min” is the minimum function, and “var” is the variancecalculation:

$\begin{matrix}{\sigma^{2} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}\left( {x_{i} - \overset{\_}{x}} \right)^{2}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

In another example, any number of desired antennas in an array,including all of the antennas, can be selected and their respectivephases, or timing, adjusted to create a signal to feed a receiver. Forexample, using the example above of a four-antenna array, if a signal istransmitted and received by a first antenna in the array, it will have afirst phase shift depending, at least in part, on the path the signaltravels to the antenna. If a second antenna of the array receives thetransmitted signal, and the signal follows a different path than thesignal to the first antenna, then the signal received at the secondantenna will have a second phase shift that is typically different thanthe first phase shift. If an omni directional antenna were used in thisexample, the differing phase shifts of the multipath signals receivedwould cause fades. Using the antenna system described herein, adjustmentof the phases, or timing, of signals received at different antennas canresult in a signal, to feed to the receiver, that is of better qualityand reduces, or eliminates, fades.

In one embodiment, timing/phase correction can be applied. In oneexample, alignment is performed on combined signals to reduce fadescaused by phase mismatch. Rough timing correction can be calculated fromthe location of the correlation peak in the time domain; phasedetection/correction can be performed in the frequency domain. Bothcalculations can be done on the amble signal.

Returning to the example of FIG. 5, the antenna system includes fourantennas 502-508 and two radios 512 and 520. In other embodimentsdifferent numbers of antennas and radios can be used. In addition, thedata interface 514 can receive baseband data and convert it into datapackets to be communicated to another system (not shown). Likewise, thedata interface 514 can receive data packets from another system andconvert it to baseband data to be communicated to the radio modules 512and 520. In addition, the data interface 514 can be configured tocommunicate baseband data, intermediate frequency (IF) signals, or RFsignals, to and from another system.

In other embodiments, other forms of the signals can be provided to areceiver in another system. For example, if it is desired to provide“raw” RF signals from the antennas to the receiver, the antenna signals550 and 552 can be communicated from the antenna switch matrix 510 tothe other system. This example illustrates an example where there aretwo raw RF signals communicated to the other system. In other examples,any desired number of raw RF signals can be provided to another system.In addition, if it is desired to provide baseband data from selectedantennas to another system, baseband signals 560 and 562 can be providedto the other system. In the example of FIG. 5, baseband signals from thetwo radio modules 510 and 520 are provided to the other system, in otherexamples with additional radio modules, additional baseband signals canbe provided to another system. In yet another example, after the antennasignal has been transformed from the time domain into the frequencydomain, such as in the radio modules 510 and 520, the “raw” frequencydomain signals 570 and 572 can be provided to the other system. In alike manner, raw RF signals, baseband signals, or raw frequency domainsignals may also be received from a transmitter in the other system andused to radiate signals from selected antennas 502-508.

In one example, during an operational phase, a first antenna may be incommunication with the first radio module 510, and a second antenna maybe in communication with the second radio module 520. The antennasignals may be demodulated by the radio modules 510 and 520 and basebanddata may be communicated to the data interface 514 where the two antennasignals are combined and a combined signal output from the datainterface 514. The combined signal output by the data interface 514 canbe baseband data, intermediate frequency (IF) signals, or RF signals.

Replacement of a conventional antenna with an antenna system such as theantenna system 500 in FIG. 5 can improve the performance of existingsystems. For example, if an existing system uses an omni-directionalantenna, the omni-directional antenna can be replaced with amulti-antenna system that can select antennas that provide improvedperformance over the omni-directional antenna. Referring to FIG. 5, theinterface between the antenna system 500 and an existing system can beat different points in the signal flow. For example, data can becommunicated between the antenna system 500 and the existing system atthe data interface 514. In addition, baseband data can be communicatedbetween the radio modules 512 and 520 and the existing system. LikewiseRF signals can be communicated between the antenna switch matrix 510 andthe existing system.

FIG. 6 is a flow diagram illustrating selection of antennas in amulti-antenna array. Flow begins in block 600. Flow then continues tobock 602 where it is determined if it is desired to bypass the automaticselection process. If it is desired to bypass the automatic selectionprocess flow continues to block 604 where antennas are manuallyselected. And flow continues to block 606 and stops.

Returning to block 602, if it is desired to automatically selectantennas flow continues to block 608 and the first antenna is selected.Flow then continues to block 610 where characteristics of signalsreceived by the selected antenna are determined. Flow continues to block614 where it is determined if all of the desired antennas have beenevaluated. If all of the desired antennas have not been ranked, thenflow continues to block 616 where another antenna is selected and flowcontinues to block 610. Returning to block 614, if all of the desiredantennas have been ranked, flow continues to block 618 and thecharacteristics of various combinations of antennas are combined and thecombinations evaluated and ranked. Flow then continues to block 620, andin response to the ranking, a preferred combination of antenna elementsis selected for an operation phase. Flow then continues to block 606 andstops.

Another example illustrating advantages to using frequency domaincombining is illustrated below. Table 4 illustrates an example set ofvalues representing magnitudes of the bins sampled from four antennas,A-D.

TABLE 4 Bin Magnitudes A 50 55 48 60 72 60 50 40 30 20 25 27 B 30 20 2527 28 29 35 35 40 51 52 60 C 60 70 66 65 48 47 46 50 30 20 25 27 D 30 2025 27 28 29 35 35 40 51 52 35

Table 5 shows values illustrating a measure of performance for variouscombinations of the antennas. In this example the values of thecombinations are determined by adding the bin magnitudes forcombinations of two antennas and then averaging the magnitudes. Forexample, the combination of antennas A and B in the first bin is(50+30)/2=40. Other techniques for combining the bin magnitudes are alsopossible.

TABLE 5 A + B 40 37.5 36.5 43.5 50 44.5 42.5 37.5 35 35.5 38.5 43.5 A +C 55 62.5 57 62.5 60 53.5 48 45 30 20 25 27 A + D 40 37.5 36.5 43.5 5044.5 42.5 37.5 35 35.5 38.5 31 B + C 45 45 45.5 46 38 38 40.5 42.5 3535.5 38.5 43.5 B + D 30 20 25 27 28 29 35 35 40 51 52 47.5 C + D 45 4545.5 46 38 38 40.5 42.5 35 35.5 38.5 31

Various statistical techniques can be used to evaluate the combinationsto determine, for example, a ranking of the various combinations. Table6 below illustrates an example analysis of the combinations illustratedin Table 5.

TABLE 6 Var Min Max Avg A + B 36.6 35 58.5 46.2 A + C 134 20 62.5 46.2A + D 96.3 31 71 46.2 B + C 74.4 35 72 46.2 B + D 134 20 62.5 46.2 C + D72.3 31 62.5 46.2

FIG. 7 is a set of charts graphically illustrating the relative“performance” of the individual antennas listed in Table 5. As shown inFIG. 7, the bin magnitudes for antenna A 702, antenna B 704, antenna C706, and antenna D 708 are plotted.

FIG. 8 is a set of charts graphically illustrating the relative“performance” of the various combinations of antennas. As shown in FIG.8, there is a chart for the combinations of antennas A and B 802,antennas A and C 804, antennas A and D 806, antennas B and C 808,antennas B and D 810, and antennas C and D 812. Inspection of the chartsof FIG. 8 shows that the combination of antennas A and B 802 is theflattest response across all of the bins. Further, the combination ofantennas A and B 802 is flatter than either antenna A or B individually,702 and 704 in FIG. 7 respectively. This example illustrates graphicallyhow the combination of antennas can have preferred performancecharacteristics over individual antennas that make up the combination.

Various illustrative implementations of the present invention have beendescribed. However, one of ordinary skill in the art will see thatadditional implementations are also possible and within the scope of thepresent invention. For example, while the above description describesspecific examples for determining which antennas within an array ofantennas to use for radio communication by a control module, theseaspects can be performed by other types of controllers and be within thescope herein.

Accordingly, the present invention is not limited to only thoseimplementations described above. Those of skill in the art willappreciate that the various illustrative modules and method stepsdescribed in connection with the above described figures and theimplementations disclosed herein can often be implemented as electronichardware, software, firmware or combinations of the foregoing. Toclearly illustrate this interchangeability of hardware and software,various illustrative modules and method steps have been described abovegenerally in terms of their functionality. Whether such functionality isimplemented as hardware or software depends upon the particularapplication and design constraints imposed on the overall system.Skilled persons can implement the described functionality in varyingways for each particular application, but such implementation decisionsshould not be interpreted as causing a departure from the scope of theinvention. In addition, the grouping of functions within a module orstep is for ease of description. Specific functions can be moved fromone module or step to another without departing from the invention.

Moreover, the various illustrative modules and method steps described inconnection with the implementations disclosed herein can be implementedor performed with a general purpose processor, a digital signalprocessor (“DSP”), an application specific integrated circuit (“ASIC”),field programmable gate array (“FPGA”) or other programmable logicdevice, discrete gate or transistor logic, discrete hardware components,or any combination thereof designed to perform the functions describedherein. A general-purpose processor can be a microprocessor, but in thealternative, the processor can be any processor, controller,microcontroller, or state machine. A processor can also be implementedas a combination of computing devices, for example, a combination of aDSP and a microprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

Additionally, the steps of a method or algorithm described in connectionwith the implementations disclosed herein can be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. A software module can reside in RAM memory,flash memory, ROM memory, EPROM memory, EEPROM memory, registers, harddisk, a removable disk, a CD-ROM, or any other form of storage mediumincluding a network storage medium. An exemplary storage medium can becoupled to the processor such the processor can read information from,and write information to, the storage medium. In the alternative, thestorage medium can be integral to the processor. The processor and thestorage medium can also reside in an ASIC.

The above description of the disclosed implementations is provided toenable any person skilled in the art to make or use the invention.Various modifications to these implementations will be readily apparentto those skilled in the art, and the generic principles described hereincan be applied to other implementations without departing from thespirit or scope of the invention. Thus, it is to be understood that thedescription and drawings presented herein represent exampleimplementations of the invention and are therefore representative of thesubject matter which is broadly contemplated by the present invention.Thus, the invention is not intended to be limited to the embodimentsshow herein, but is to be accorded the widest scope consistent with theappended claims.

1. An antenna system for use in an orthogonal frequency divisionmultiplexing (OFDM) communication system, the antenna system comprising:a plurality of antennas; a switch matrix in communication with theplurality of antennas and adapted to couple selected antennas to aplurality of switch outputs; a radio module in communication with afirst switch output and adapted to receive OFDM radio frequency signals;a measurement module in communication with the radio module, and adaptedto measure the bin magnitudes of the OFDM radio frequency signalsreceived by the radio module; and a control module adapted to controlselection of which of the plurality of antennas is coupled to the firstswitch output, wherein during a selection phase the control moduleselectively couples a desired antenna to the first switch output andreceives the measured bin magnitudes of the OFDM radio frequency signalreceived by the selected antenna, the control module continues selectingand receiving measurements from a desired number of the plurality ofantennas, then the control module combines the measurements fromcombinations of the antennas and determines a desired combination ofantennas for use during an operational phase, wherein the desiredcombination of antennas has preferred performance characteristics overindividual antennas that make up the desired combination, and whereinthe control module during the selection phase selects antennas one at atime and ranks the antennas based upon the measured characteristics. 2.The antenna system of claim 1, wherein the combinations of antennasincludes measurements from at least four antennas.
 3. The antenna systemof claim 1 further comprising: a second switch output; and a secondradio module in communication with the second switch output, wherein thecontrol module selects one antenna to be communicatively coupled to theradio module and a second antenna to be communicatively coupled to thesecond radio module, wherein the OFDM radio signal received by thesecond radio are communicated to the measurement module.
 4. The antennasystem of claim 3, wherein measurements of the OFDM radio signalreceived by the radio module and the second radio module are madesimultaneously by the measurement module.
 5. A method of selecting a setof antennas from a plurality of antennas for use with a radio systemhaving a plurality of radios operating in an orthogonal frequencydivision multiplexing (OFDM) communication system, the methodcomprising: selecting one antenna within an array of antennas; measuringthe bin magnitudes of OFDM radio signals received at the antenna;repeating selecting, and measuring, on a desired number of antennaswithin the array, wherein the selecting and measuring of antennas isperformed one at a time; combining the measurements of the measured binmagnitudes from the antennas into a plurality of combinationmeasurements; ranking the antenna combinations based upon thecombination measurements; and selecting a desired combination ofantennas for use during operation of the radio system, wherein thedesired combination of antennas has preferred performancecharacteristics over individual antennas that make up the desiredcombination.
 6. The method of claim 5, wherein the measuring occurs inthe frequency domain.
 7. The method of claim 5, wherein measuring binmagnitudes of the OFDM radio signal is performed in the frequencydomain.
 8. The method of claim 5, wherein combining the measurements ofthe measured bin magnitudes from the antennas into a plurality ofcombination measurements further comprises combining signals receivedfrom two or more antennas and then measuring a degree of correlation ofan amble in the combined signal to a known amble.
 9. The method of claim5, wherein measuring further comprises measuring a signal to noiseratio.
 10. The method of claim 5, wherein measuring further comprisesmeasuring a carrier to signal ratio.
 11. The method of claim 5, whereinmeasuring further comprises a signal fade measurement.
 12. The method ofclaim 5, further comprising: determining a phase of signals receivedfrom the desired number of antennas; and adjusting the phase of thesignals of the selected set of antennas.
 13. The method of claim 5,further comprising: determining a timing of signals received from thedesired number of antennas; and adjusting the timing of the signals ofthe selected set of antennas.
 14. An antenna system comprising: an arrayof antennas; a switch matrix in communication with the array of antennasand adapted to couple selected antennas to a plurality of switchoutputs; a radio module in communication with a first switch output andadapted to receive OFDM radio frequency signals; a measurement module incommunication with the radio module, and adapted to measurecharacteristics of the OFDM radio frequency signals received by theradio module, wherein the characteristics include the bin magnitudes ofthe OFDM radio frequency signals; and a control module adapted tocontrol the switch matrix to selectively couple a desired antenna to thefirst switch output and receives the measured characteristics of theradio frequency signal received by the selected antenna, the controlmodule continues selecting and receiving measurements from a desirednumber of the plurality of antennas, then combining the measurements ofthe plurality of antennas into a plurality of combination measurementsand based upon the combination measurements, providing independentcommunication streams from selected combinations of antennas to areceiver, wherein the control module, during the selection phase,selects antennas one at a time; and wherein the selected combinations ofantennas have preferred performance characteristics over individualantennas that make up the selected combinations.
 15. The antenna systemof claim 14, wherein the measuring occurs in the frequency domain. 16.The antenna system of claim 14, wherein measuring characteristics of theradio signal further comprises measuring a degree of correlation of anamble in the signal to a known amble.
 17. The antenna system of claim14, wherein measuring characteristics of the radio signal furthercomprise measuring a degree of correlation of an amble in the signal toa known amble.
 18. The antenna system of claim 14, wherein measuringcharacteristics of the radio signal further comprises an error vectormeasurement.
 19. The antenna system of claim 14, wherein measuringcharacteristics or the radio signal further comprises measuring a signalto noise ratio.
 20. The antenna system of claim 14, wherein measuringcharacteristics of the radio signal further comprises measuring acarrier to signal ratio.
 21. The antenna system of claim 14, whereincombining the measurements of the plurality of antennas into a pluralityof combination measurements comprises combing the measurements for pairsof antennas.
 22. The antenna system of claim 14, wherein measuringcharacteristics of the radio signal further comprises measuring thephase of the radio signal.
 23. The antenna system of claim 14, whereinmeasuring the characteristics of the radio signal further comprisesmeasuring the timing of the radio signal.
 24. A method of selecting aset of antenna from an array of antennas, the method comprising:selecting an antenna within an array of antennas; measuringcharacteristics of an orthogonal frequency division multiplexed (OFDM)radio signals received at the selected antenna, wherein thecharacteristics include the bin magnitudes of the OFDM radio frequencysignals; repeating selecting, and measuring, on a desired number ofantennas within the array, wherein the selecting and measuring ofantennas is performed one at a time; combining the measurements ofselected antennas into a plurality of combination measurements; rankingthe combinations of antenna base upon the combination measurements; andselecting combinations of antennas to thereby provide independentcommunication streams from the combinations to a receiver, wherein theselected combinations of antennas have preferred performancecharacteristics over individual antennas that make up the selectedcombinations.
 25. The method of claim 24, wherein the measuring occursin the frequency domain.
 26. The method of claim 24, wherein measuringcharacteristics of the radio signal further comprises measuring a degreeor correlation of an amble in the signal to a known amble.
 27. Themethod of claim 24, wherein measuring characteristics of the radiosignal further comprise combining signals received from two or moreantennas and then measuring a degree of correlation of an amble in thecombined signal to a known amble.
 28. The method of claim 24, whereinmeasuring characteristics of the radio signal further comprises an errorvector measurement.
 29. The method of claim 24, wherein measuringcharacteristics of the radio signal further comprises measuring a signalto noise ratio.
 30. The method of claim 24, wherein measuringcharacteristics of the radio signal further comprises measuring acarrier to signal ratio.
 31. The method of claim 24, wherein measuringcharacteristics of the radio signal further comprises a signal fademeasurement.
 32. The method of claim 24, further comprising: determininga phase of signals received from the desired number of antennas; andadjusting the phase of the signals of the selected set of antennas. 33.The method of claim 24, further comprising: determining a timing ofsignals received from the desired number of antennas; and adjusting thetiming of the signals of the selected set of antennas.
 34. An antennasystem for use in an orthogonal frequency division multiplexed (OFDM)communication system, the antenna array comprising: an array ofantennas; a switch matrix in communication with the array of antennasand adapted to couple selected antennas to a plurality of switchoutputs; a radio module in communication with a first switch output andadapted to receive OFDM radio frequency signals; a measurement module incommunication with the radio module, and adapted to measurecharacteristics of the OFDM radio frequency signals received by theradio module, the characteristics including the bin magnitudes of thereceived OFDM radio frequency signals; and a control module adapted tocontrol the switch matrix to selectively couple a desired antenna to thefirst switch output and receives the measured characteristics of theradio frequency signal received by the selected antenna, the controlmodule continues selecting and receiving measurements from a desirednumber of the plurality of antennas, then combining the measurements ofthe plurality of antennas into a plurality of combination measurementsand based upon the combination measurements, providing independentcommunication streams from selected antennas to a receiver, wherein thecontrol module during the selection phase selects antennas one at atime; and wherein the communication streams from selected antennas havepreferred performance characteristics over communication streams fromindividual antennas.
 35. The antenna system of claim 34, wherein theindependent communication streams are baseband signals.
 36. The antennasystem of claim 34, wherein the independent communication streams are RFsignals.
 37. The antenna system of claim 34, wherein the independentcommunication streams are frequency domain signals.
 38. A method ofselecting a set of antenna from an array of antennas, the methodcomprising: selecting an antenna within an array of antennas; measuringcharacteristics of orthogonal frequency division multiplexed (OFDM)radio signals received at the selected antenna, where thecharacteristics comprise bin magnitudes; repeating selecting, andmeasuring, on a desired number of antennas within the array, wherein theselecting and measuring of antennas is performed one at a time;combining the measurements of selected pairs antennas into a pluralityof combination measurements; ranking the combinations of antenna baseupon the combination measurements; and selecting combinations ofantennas to thereby provide independent communication streams to areceiver, wherein the selected combinations of antennas have preferredperformance characteristics over individual antennas that make up theselected combinations.
 39. The antenna system of claim 38, whereinproviding independent communication streams comprises providing basebandsignals.
 40. The antenna system of claim 38, wherein providingindependent communication streams comprises providing RF signals. 41.The antenna system of claim 38, wherein providing independentcommunication streams comprises providing frequency domain signals.